1. Field of the Invention
This invention relates generally to a cathodoluminescence (CL) detection device and more particularly to a CL detection device utilizing a concave mirror of semi-ellipsoidal shape with high ellipticity. The mirror material is of low effective atomic number, which ensures that most of the backscattered electrons (BSEs) emitted simultaneously with CL emissions are absorbed.
2. Brief Description of the Prior Art
Cathodoluminescence is the emission of radiation from materials due to an incident electron beam. The phenomenon can be used to provide information on the properties of materials and devices. When the beam strikes the specimen, the spectrum of the emissions from the specimen can be obtained with a CL detection device fitted with a wavelength selector and a suitable emission sensitive detector. Monochromatic and panchromatic CL images can be obtained with an appropriate image recording instrument. For monochromatic imaging, a wavelength selector is inserted to select a particular wavelength or a range of wavelengths of emissions with which to form the image. For panchromatic imaging, the wavelength selector is bypassed.
Due to the difficulties in mirror design, most existing CL detectors are unable to perform both spectroscopy and microscopy. For example, the CL detectors described by Phang, et al. in U.S. Pat. No. 5,264,704 and Braglia, et al. in U.S. Pat. No. 5,010,253 lack spectroscopic abilities due to their direct detection of emitted radiation without passing through a wavelength selector. A further disadvantage of these CL detectors is that they have to use solid state detectors which are less sensitive than a photomultiplier tube (PMT).
The most important issue for spectroscopic CL applications lies in the efficiency of the detector in transmitting the collected radiation to a radiation sensitive device. The way in which the radiation is collected and transmitted seriously affects the sensitivity as well as the flexibility of the CL detector. Efficient transmission of radiation from the mirror to the radiation guide and the wavelength selector requires the incoming radiation to be confined within a small cone with an angle which is less than a critical value. Radiation outside the cone is not collected and losses in transmission will be incurred. The problems of designing a CL detector with both spectroscopic and imaging capabilities without losing collection and transmission efficiencies are described by Steyn, et al. in 107 J. Microscopy Pt. 2, 107-128 (July, 1976) ("An efficient spectroscopic detection system for cathodoluminescence mode scanning electron microscopy (SEM)"). The systems described by Vahala, et al. (U.S. Pat. No. 4,929,041), Schafer, et al. (U.S. Pat. No. 4,900,932) and Horl, et al. (U.S. Pat. No. 3,790,781) relate to CL detectors having this problem of transmission inefficiency arising from poor coupling from the mirror to the radiation guide and the wavelength selector. The systems of Vahala, et al. and Horl, et al. are also limited in terms of the specimen size which can be observed. Furthermore, these detectors are not easily retractable from the path of the electron beam and therefore do not allow other modes of electron beam imaging without extensive modifications to the system.
The height of the mirror in a CL system limits the minimum distance between the specimen and the electron beam generator unit. This has an impact on the spatial resolution performance of the system. For this reason, in CL observations requiring high spatial resolution, the height of the mirror must be kept small. Apart from the CL detector described by Braglia, et al., all of the patents discussed above are severely limited in this aspect.